It is well known that adaptive antenna arrays offer the potential for significant capacity improvements in interference-limited cellular wireless systems. See, e.g., J. Liberti, T. Rappaport, “Analytical results for capacity improvements in CDMA”, IEEE Transactions on Vehicular Technology, vol. 43, pp. 680-690, 1994, and J. Winters et al., “The impact of antenna diversity on the capacity of wireless communication systems,” IEEE Transactions on Communications, vol. 42, pp.1740-1751, 1994.
In cellular wireless systems with adaptive antenna arrays, the multiple antennas of the array are typically deployed at the base station of each cell, and the signals transmitted or received by the antennas are linearly combined with certain complex weights. Different antenna weights are used to extract the signals transmitted to or received from different mobile stations within the cell. By properly adjusting the antenna weights, the multiple antennas can improve the signal-to-interference ratio (SIR) through beamforming, interference cancellation and receive diversity. However, realizing such capacity improvements demands proper adaptation of the antenna weights, and developing suitable adaptation algorithms can be a challenging problem in modem wireless systems with high mobility or bursty data traffic.
Several antenna array algorithms have been developed for traditional wireless technologies, such as code division multiple access (CDMA) and time division multiple access (TDMA). The key to any adaptive antenna array algorithm is for the base station to first isolate the signals from each mobile station within the cell, and then tune the antenna weights to capture those signals.
In CDMA systems, the signals from the mobile stations can be isolated by their unique spreading codes, as described in, e.g., Z. Rong et al., “Simulation of multitarget adaptive antenna algorithms,” Proc. Vehicular Technology Conference, pp. 1-5, 1997.
For TDMA systems, the signal from each mobile station arrives in a unique time slot. Additionally, in TDMA standards such as IS-54 and IS-136, a known synchronization sequence within each time slot can also be exploited for antenna adaptation. See, e.g., G. Bottomley and K. Jamal, “Adaptive antenna arrays and MLSE equalization,” Proc. Vehicular Technology Conference, pp. 50-54, 1995, and J. Winters, “Signal Acquisition and tracking with adaptive antenna arrays in the digital mobile radio system IS-54 with flat fading,” IEEE Transactions on Communications, vol.42, pp.377-384, 1993.
Unfortunately, similar adaptive antenna algorithms have not been developed for cellular wireless systems based on orthogonal frequency division multiplexing with spread-spectrum multi-access (OFDM-SSMA). OFDM-SSMA is a novel technology for cellular wireless systems that is described in U.S. patent application Ser. No. 09/267,471, entitled “Orthogonal frequency division multiplexing based spread spectrum multiple access,” and U.S. patent application Ser. No. 09/266,370, entitled “Orthogonal frequency division multiplexing based spread spectrum multiple access,” both filed Mar. 11, 1999 in the name of inventors R. Laroia, J. Li and M. Vanderveen, and incorporated by reference herein.
In OFDM-SSMA, both the uplink and downlink bandwidth are divided into a number of equally spaced tones that are re-used in all cells. Each mobile station is allocated one or more of these tones, with different mobile stations in the same cell using different tones. For frequency diversity and interference averaging, the tones assignments are changed, or hopped, from symbol to symbol. OFDM-SSMA offers the benefits of traditional OFDM such as resistance to delay spread along with other benefits such as in-cell orthogonality, universal frequency re-use and frequency diversity.
Adaptive antenna algorithms for traditional multi-access technologies such as CDMA and TDMA cannot be easily adapted to OFDM-SSMA. Since OFDM-SSMA does not use code division or time division multiple access, the signals from the mobile stations cannot be isolated by any unique spreading code or time slot. In addition, signals from OFDM-SSMA mobile stations do not typically contain any training symbols or pilots. Therefore, the IS-54 and IS-136 pilot-based methods mentioned above cannot be used in OFDM-SSMA without adding additional training symbols. Also, since OFDM-SSMA signals are inherently frequency hopping, the channel is constantly changing. However, traditional adaptive algorithms usually require channel coherence over the training period. Consequently, if the benefits of antenna arrays are to be realized for OFDM-SSMA systems, new adaptive antenna algorithms must be developed.